Hollow plastic molded articles used for the storage or transport of liquid substances are widely used for daily living or industrial field. In particular, in automobile parts, a hollow plastic molded article to be used as a fuel tank is taking the place of a conventional metal material-made fuel tank. Furthermore, at present, plastics are a material most frequently used for production of a fuel can of inflammable liquids, noxious substances or the like and a carriage container such as plastic bottles or the like. Since plastic-made containers and tanks are low in a weight/voltage ratio as compared with those made of a metal material, they have such merits that it is possible to reduce the weight, corrosion such as rust and the like hardly occurs, and impact resistance is good, and they are acquiring wide uses more and more.
In many cases, the hollow plastic molded articles are mainly obtained by means of blow molding from high-density polyethylene (HDPE). Polyethylene containers are not sufficient in an intercepting action (barrier action) for mainly suppressing penetration of the contents toward the outside. In the case of automobiles, in view of the fact that a volatile substance such as a fuel and the like will be an environmental pollutant, severe legal regulations are imposed regarding the exhaust. Since the barrier action of polyethylene against the penetration of a volatile substance is low, grapping for reducing the penetration by adopting more measures is taken. A measure which is the most important for attaining this is a fluorine treatment of the surface of a container, or introduction of a battier layer made of a polar barrier plastic. The barrier layer of this kind is introduced as a wall within a container by a technology known as multi-layer co-extrusion blow molding.
In almost all cases, the barrier layer is low in mechanical strength in co-extrusion blow molding. In barrier layer-containing containers, the impact properties are more likely influences particularly at low temperatures, as compared with those of barrier layer-free non-coated high-density polyethylene containers.
In plastic fuel tanks obtained from polyethylene, attention should be paid especially to requirements which will be problems. Since the plastic fuel tanks are classified as an important safety part for ensuring safety of automobiles, they are required to provide with high levels especially with respect to mechanical strength, durability and impact resistance, and the development of materials for enhancing these matters to sufficiently high levels is desired.
As to the hollow plastic molded articles, for example, there are proposed hollow plastic molded articles having one or more layers made of polyethylene obtained using a fluorine-modified chromium catalyst (see Patent Document 1). But, as compared with the case where fluorine modification is not performed, when a fluorine-modified chromium catalyst is used, a molecular weight distribution of obtained polyethylene becomes narrow, and therefore, there are brought results that the durability does not sufficiently satisfy a level as a hollow plastic molded article, particularly a fuel tank for automobiles.
Also, as to polyethylene, there is proposed a method for producing polyethylene suitable for blow molded articles, particularly large blow molded articles, by performing polymerization using a trialkylaluminum compound-supported chromium catalyst while allowing hydrogen to coexist (see Patent Document 2). Also, the subject patent document also discloses a method for producing polyethylene using a dialkylaluminum alkoxide compound-supported chromium catalyst (Comparative Example 13). However, polyethylene suitable for hollow plastic molded articles, particularly a fuel tank for automobiles is not disclosed, and it is hard to say that a fuel tank for automobiles with a sufficient level of durability can be produced.
Also, there is proposed a method for producing polyethylene using a chromium catalyst by adding an organoaluminum compound as a co-catalyst to a polymerization system (see Patent Document 3). The subject patent document also discloses a method for producing polyethylene using a trialkylaluminum and/or dialkylaluminum alkoxide compound-supported chromium catalyst (Examples Nos. 2 to 6). However, polyethylene suitable for hollow plastic molded articles, particularly a fuel tank for automobiles is not disclosed. Furthermore, the subject patent document also discloses a method for producing polyethylene using a titania-containing chromium catalyst obtained by impregnating titanium tetraisopropoxide onto silica before activation of a Cr catalyst to achieve calcination activation. But, polyethylene obtained at that time tends to be low in its impact strength.
Also, there is proposed a method for producing polyethylene using a trialkylaluminum and/or dialkylaluminum alkoxide-supported chromium catalyst (see Patent Document 4). However, polyethylene suitable for hollow plastic molded articles, particularly a fuel tank for automobiles is not disclosed.
Also, there is proposed a catalyst for ethylene based polymerization composed of a solid chromium catalyst component prepared by allowing an inorganic oxide carrier to support a chromium compound in which at least a part of chromium atoms can be converted into a hexavalent chromium atom thereon by performing calcination activation in a non-reducing atmosphere, a dialkylaluminum functional group-containing alkoxide and a trialkylaluminum (see Patent Document 5). The subject patent document also discloses an ethylene based polymer for blow molded articles exhibiting excellent creep resistance and ESCR and having an HLMFR of from 1 to 100 g/10 min and a density of from 0.935 to 0.955 g/cm3. The subject patent document also discloses a method for producing polyethylene using a trialkylaluminum and/or dialkylaluminum alkoxide compound-supported chromium catalyst (Comparative Examples 3 and 13).
However, the subject patent document neither suggests nor discloses polyethylene suitable for hollow plastic molded articles, particularly a fuel tank for automobiles with excellent impact resistance.
Also, there is proposed a method for producing an ethylene based polymer using a chromium catalyst prepared by allowing a chromium compound-supported inorganic oxide carrier prepared by supporting a chromium compound on an inorganic oxide carrier and performing calcination activation in a non-reducing atmosphere, to convert at least a part of chromium atoms into a hexavalent chromium atom, thereby supporting a specified organoaluminum compound (alkoxides, siloxides, phenoxides, etc.) thereon in an inert hydrocarbon solvent (see Patent Document 6). The subject patent document discloses an ethylene based polymer having an excellent balance between environmental stress crack resistance (ESCR) and stiffness.
Also, there is proposed an ethylene based polymer producing catalyst which is characterized by being composed of a chromium catalyst prepared by supporting a chromium compound on an inorganic oxide carrier and performing calcination activation in a non-reducing atmosphere, thereby converting at least a part of chromium atoms into a hexavalent chromium atom and a specified organoaluminum compound (alkoxides, siloxides, phenoxides, etc.) (see Patent Document 7). The subject patent document discloses an ethylene based polymer exhibiting excellent ESCR or creep resistance.
Furthermore, there is proposed a method for producing an ethylene based polymer which is characterized in that in multistage polymerization of ethylene alone or copolymerization of ethylene and an α-olefin having from 3 to 8 carbon atoms continuously in a plurality of polymerization reactors connected in series by using a chromium catalyst prepared by supporting a chromium compound on an inorganic oxide carrier and performing calcination activation in a non-reducing atmosphere, thereby converting at least a part of chromium atoms into a hexavalent chromium atom, and a specified organoaluminum compound (alkoxides, siloxides, phenoxides, etc.) is introduced into any one or all of the polymerization reactors (see Patent Document 8). The subject patent document discloses an ethylene based polymer exhibiting excellent environmental stress crack resistance (ESCR) and creep resistance. However, in the foregoing patent document, though an ethylene based polymer having a molecular weight distribution (Mw/Mn) of 20.9 (working examples) is disclosed, polyethylene suitable for hollow plastic molded articles, particularly a fuel tank for automobiles with excellent impact resistance is neither suggested nor disclosed.
Also, there is proposed a catalyst for ethylene based polymerization prepared by allowing a fluorinated chromium compound in which at least a part of chromium atoms can be converted into a hexavalent chromium atom by performing activation in a non-reducing atmosphere, to support a specified organic boron compound thereon (see Patent Document 9). The subject patent document also discloses a method for producing polyethylene using a trialkylaluminum and/or dialkylaluminum alkoxide compound-supported chromium catalyst (Comparative Examples 6 and 8). However, the subject patent document neither suggests nor discloses polyethylene suitable for hollow plastic molded articles, particularly a fuel tank for automobiles.
In addition to the above, as commercially available polyethylene used for a fuel tank for automobiles, for example, HB111R, manufactured by Japan Polyethylene Corporation; 4261AG, manufactured by Basell Polyolefins; and the like are known. Though these are a material which meets severe requirements of automobile manufactures and is acclaimed in the market, it may not be always said that they are on a sufficiently high level regarding a balance between durability and stiffness and issues of impact resistance and moldability.
Under such circumstances, there are desired polyethylene overcoming the problems of polyethylene so far, exhibiting excellent moldability and durability and having an excellent balance between impact resistance and stiffness and in particular, capable of realizing excellent high stiffness; and polyethylene suitable for hollow plastic molded articles, particularly fuel tanks with high performances.